(1) Field of the Invention
This invention relates to compressors, and more particularly to screw-type compressors.
(2) Description of the Related Art
Screw-type compressors are commonly used in air conditioning and refrigeration applications. In such a compressor, intermeshed male and female lobed rotors or screws are rotated about their axes to pump the refrigerant from a low pressure inlet end to a high pressure outlet end. During rotation, sequential lobes of the male rotor serve as pistons driving refrigerant downstream and compressing it within the space (compression pocket) between an adjacent pair of female rotor lobes and the housing. Likewise sequential lobes of the female rotor produce compression of refrigerant within a male rotor compression pocket between an adjacent pair of male rotor lobes and the housing. In one implementation, the male rotor is coaxial with an electric driving motor and is supported by bearings on inlet and outlet sides of its lobed working portion. There may be multiple female rotors engaged to a given male rotor or vice versa. With such a compressor, male and female compression pockets may also have multiple inlet and outlet ports.
When a compression pocket is exposed to an inlet port, the refrigerant enters the pocket essentially at suction pressure. As the pocket continues to rotate, at some point during its rotation, the pocket is no longer in communication with the inlet port and the flow of refrigerant to the pocket is cut off. Typically the inlet port geometry is arranged in such a way that the flow of refrigerant is cut off at the time in the cycle when the pocket volume reaches its maximum value. Typically the inlet port geometry is such that both male and female compression pockets are cut off at the same time. The inlet port is typically a combination of an axial port and a radial port. After the inlet port is closed, the refrigerant is compressed as the pockets continue to rotate and their volume is reduced. At some point during the rotation, each compression pocket intersects the associated outlet port and the closed compression process terminates. Typically outlet port geometry is such that both male and female pockets are exposed to the outlet port at the same time. As with the inlet port, the outlet port is normally a combination of an axial port and a radial port. By combining axial and radial ports into one design configuration, the overall combined port area is increased, minimizing throttling losses associated with pressure drop through a finite port opening area.
Accordingly, one aspect of the invention involves a compressor having a housing containing male and female rotors with intermeshed screw-type bodies and held for rotation about respective axes. The housing has first and second portions respectively cooperating with the male and female rotors to respectively define inlet ports to respective male and female compression pockets. The housing has third and fourth portions respectively cooperating with the male and female rotors to respectively define outlet ports from the respective male and female compression pockets. The housing portions are positioned so that, with the male rotor revolving at a given speed, the male and female compression pockets are closed for unequal male and female durations.
In various implementations, the male duration may be less than the female duration. This may be by an exemplary time of between 0.5% and 20% of a cycle time at the speed. If the respective inlet ports are closed simultaneously, the male outlet port may be opened before the female outlet port. If the respective outlet ports are opened simultaneously, the female inlet port may be closed before the male inlet port. The male inlet port may close after the female inlet port while the male outlet port may open before the female outlet port. Volume indices of the male and female compression pockets may be within 5% of each other.
Another aspect of the invention is a method for engineering or reengineering a design of a compressor having intermeshed male and female lobed rotors. An initial design has initial male and female compression pocket volume indices and peak pressures under given operating conditions. The relative opening or closing time of male and female compression pockets in an embodiment of the design is varied. Volume indices or peak pressures reflecting the varying are observed. The varying and observations are repeated until the volume indices and/or peak pressures associated with the particular revised design are within a desired proximity, less than a difference between the initial male and female compression pocket volume indices and/or peak pressures. The embodiment may be a computer simulation. The volume indices may initially be mismatched by a factor of greater than 10% and may be corrected to within 5%.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.